diff --git a/crates/coremem/src/sim/spirv/gpu.rs b/crates/coremem/src/sim/spirv/gpu.rs
index 213e9c1..e392e79 100644
--- a/crates/coremem/src/sim/spirv/gpu.rs
+++ b/crates/coremem/src/sim/spirv/gpu.rs
@@ -59,7 +59,7 @@ where
     pub(super) fn step_spirv<S: AbstractStimulus>(&mut self, num_steps: u32, stim: &S) {
         let size = self.size();
         let (stim_cpu_e, stim_cpu_h) = self.eval_stimulus(stim);
-        let field_bytes = size.product_sum() * 12;
+        let field_bytes = size.product_sum() as usize * std::mem::size_of::<Vec3<f32>>();
 
         let sim_meta = [self.meta.clone()];
 
@@ -202,7 +202,7 @@ where
             0,
             &e_readback_buffer,
             0,
-            field_bytes as _,
+            field_bytes as u64,
         );
 
         encoder.copy_buffer_to_buffer(
@@ -210,7 +210,7 @@ where
             0,
             &h_readback_buffer,
             0,
-            field_bytes as _,
+            field_bytes as u64,
         );
 
         encoder.copy_buffer_to_buffer(
@@ -218,7 +218,7 @@ where
             0,
             &m_readback_buffer,
             0,
-            field_bytes as _,
+            field_bytes as u64,
         );
 
         queue.submit(Some(encoder.finish()));